Surface treatment of steel electrotinning stock



United States Patent 3,468,768 SURFACE TREATMENT OF STEEL ELECTROTINNING STOCK Stewart E. Ranch, Jr., Bethlehem, Pa., assignor to Bethlehem Steel Corporation, a corporation of Delaware No Drawing. Filed July 5, 1966, Ser. No. 562,483 Int. Cl. C23b 5/52, 5/04 US. Cl. 20437 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to conditioning of the surface of steel strip prior to electrotinning, and more particularly to a method of pickling the strip.

In the production of electrotinplate, the steel strip base material which has been cold rolled to gage, annealed, and usually given a temper roll, is cleaned of dirt and oil, and pickled in dilute mineral acid. After pickling, the strip is thoroughly rinsed in water and then introduced into the electrotinning bath. Pickling, preceding electrotinning, is usually performed in an aqueous solution of approximately 3 to 5 weight percent sulfuric acid.

Electrotinplate can be made from either an alkaline or an acid plating bath, the latter type gaining favor due to the greater speed at which tinplate can be produced. However, alkaline tinplate has generally been considered in the tinplate industry as having corrosion resistance superior to acid tinplate. This superiority is of particular importance in the shelf life of tin cans containing the more corrosive types of food and beverages.

The observed differences in corrosion characteristics between alkaline and acid electrotinplate have led to the establishment of certain standards, which govern the purchase of electrotinplate for specific uses.

The term Grade K is now used to designate that tinplate grade which meets requirements of can shelf life for the more corrosive acid types of food and beverages, including citrus fruits and tomatoes.

Tinplate, in order to meet Grade K requirements, must have an iron solution value (ISV) of not more than 20. Iron solution value represents the dissolved iron in micrograms in a test solution of sulfuric acid containing ammonium-thiocyanate and hydrogen peroxide, after exposure of one side of a tinplate sample of specific dimensions to 50 milliliters of the test solution in a closed jar for 2 hours at 80 F. The ISV test is the subject of a discussion in Corrosion, vol. 12, No. 9, September 1956, pp. 23-30, the test being described in Appendix BThe Iron Solution Test at p. 29.

Grade K tinplate must have, additionally, an alloytin couple (ATC) value of not more than 0.12. The alloy-tin couple value represents the current flow in microamperes per square centimeter between a tin anode and the tin-iron alloy of a tinplate sample, after immersing a tinplate sample of specific dimensions, from which the free tin has been removed, in citrus juice for 24 hours. The ATC test is discussed in Corrosion, vol. 17, No. 2, February 1961, pp. 106l14, and the test is described in Appendix A-Detailed ATC Procedure at pp. 113-114.

ISV and ATC values have relevance only in connection with what is known as fused, or flow brightened, tinplate.

In commercial practice, alkaline tinplate usually meets the foregoing requirements. The same cannot be said for Ice acid tinplate, which sometimes meets, and at other times does not meet, ISV and ATC requirements.

It is an object of this invention to provide a treatment for cold rolled, annealed steel strip, which, when subsequently electrotinned and flow brightened, will have a low iron solution value.

It is a further object to provide a treatment for the strip, which, when subsequently electrotinned and flow brightened, will have an improved ATC value.

1 have found that the above objects can be attained by pickling the cold rolled annealed strip in a controlled pickling operation in a pickling bath comprising a dilute aqueous solution of nitric acid and hydrochloric acid.

The method comprises, briefly, pickling a cold rolled annealed strip, from which dirt and oil have been removed, in an aqueous solution of from 2 to 5.5 weight percent nitric acid HNO and from 0.2 to 1.7 weight percent hydrochloric acid (100% HCl). The strip is held in the pickling solution for a matter of seconds, after which the pickling solution is thoroughly removed from the strip, and the strip is ready for the electrotinning operation.

A preferred method by which this invention can be performed is given in the following detailed description.

Cold rolled, annealed steel strip, having a gage of 0.010 inch, and a carbon content of 0.05 weight percent, is cleaned cathodically in an aqueous solution of about 3 weight percent sodium carbonate. After being thoroughly rinsed with water and passed through rubber-coated squeegee rolls, the strip is introduced into an aqueous pickling bath comprising 40 grams per liter (4 weight percent) nitric acid and 4 grams per liter (0.4 weight percent) hydrochloric acid at a bath temperature of approximately 100 F. The strip remains in the pickling bath for 5 seconds and is then withdrawn, thoroughly rinsed with water, and excess water removed by passing through rubbercoated squeegee rolls. The thus pickled and rinsed strip is ready to be introduced into an electrotinning bath to receive a coating of tin, after which the coated product is flow brightened.

By use of the pickling method just described, followed by conventional electrotinning and flow brightening, I have produced tinplate having an iron solution value not exceeding 20, and an alloy-tin couple value not exceeding 0.12.

While the primary purpose of this invention has been that of producing acid tinplate in which the iron solution value is not greater than 20, the fact that the pickling method has the additional effect of lowering alloy-tin couple values is quite significant.

Other methods have been used to lower the ATC value, principally the expedient of altering the temperature of the tin brightening, or fusing, furnace. Temperature of the fusing operation and the holding time therein, plus control of the subsequent quench, can be regulated so that acceptable ATC values can be obtained. Such control generally requires major modification of existing tin fusing furnaces and practice, and, in some instances, inclusion of a second fusing furnace in the tin line.

Addition of an extra fusing furnace, or modification of an existing one, generally calls for a considerable capital expense. By use of my invention, improved ATC values are obtained with a conventional tin fusing practice requiring no alteration in fusing equipment. At the same time, the ISV of acid tinplate produced from box annealed steel strip, heretofore erratic, will be within the acceptable range for Grade K tinplate. The average ISV of acid tinplate produced from box annealed strip, wherein the steel strip has been pickled in the conventional 5 weight percent sulfuric acid solution, is frequently above 20. The average ATC value with sulfuric acid pickling and no alteration of fusing practice is generally above 0.12.

The outstanding improvement in both ISV and ATC value obtained with the nitric-hydrochloric acid pickling method, as against results obtained with conventional sulfuric acid pickling, is shown from test results comparing the two pickling procedures.

For the comparative tests, box annealed, 0.010 inch gage blackplate having a carbon content of 0.05% was used.

Blackplate test pieces were made up in the form of cylinders. Each cylinder had a diameter of 6 inches and a height of 6 inches, the cylinder being treated as described in the test procedure given below to a height of inches.

The test procedure comprised fixing the cylindrical test piece in a vertical position on a rotating current conducting head, after which the test cylinder was first cleaned electrolytically in a proprietary alkaline cleaning solution, rinsed in water, pickled, rinsed, plated electrolytically with tin on both sides from an acid plating bath and rinsed. A number of thus treated cylinders were then cut up into panels having a dimension of 4 inches by 17.5 inches. The panels were flow brightened in an induction furnace, and from these panels, specimens were obtained upon which determinations were made for iron solution values and alloy-tin couple values.

Three series of tests were conducted, tests being performed on ten specimens in each series. In Series I, the pickling step was performed in an aqueous solution of 50 grams per liter of sulfuric acid at 130 F. In Series II and HI, the aqueous pickling solution comprised, in the first instance, 30 grams per liter nitric acid and 5 grams per liter hydrochloric acid, and in the second instance, 47.2 grams per liter nitric acid and 2.3 grams per liter hydrochloric acid. Pickling in Series II and III was performed at 104 F. The results of the tests are shown in the following table.

4 hydrochloric acid, above about 1.7%, will hinder development of good ISVs and ATC values.

overpickling may also occur if the steel strip remains in the pickling bath for periods longer than about 1 minute. The time of pickling should be limited, preferably, to not more than about 30 seconds. Good results can be obtained with a pickling time as low as 1 second, with 3 to 5 seconds being preferred.

I claim:

1. The method for the production of electrotinned strip which comprises immersing cold rolled annealed steel strip in an aqueous solution of from 2 to 5.5 weight percent of nitric acid calculated as 100% HNO and from 0.2 to 1.7 weight percent hydrochloric acid calculated as 100% HCl for a period of not more than about 60 seconds, rinsing the strip with water, electrotinning the strip and flow brightening the electrotinned strip.

2. A method according to claim 1 in which the nitric acid is present in an amount of from 3 to 5 weight percent and the hydrochloric acid is present in an amount of from 0.4 to 0.8 weight percent.

3. A method according to claim 1 in which the immersion period ranges from 1 second to 30 seconds.

4. An electrotinned carbon steel strip having its tin surface flow brightened, in which the surface of the steel substrate has been conditioned prior to electrotinning by pickling in an aqueous solution of from 2 to 5.5 weight percent nitric acid (100% HNO and from 0.2 to 1.7 weight percent hydrochloric acid (100% HCl).

5. A method for the production of electrotinned strip wherein the resultant tin coating has a low iron solution value and a low alloy-tin couple value which comprises ISV ATC micro micro- Tempera- Time, grams of amp./sq.

Test Pickling bath ture, F. seconds Fe/disc. cm.

Series I H2804, 5 g./l 130 5 48 0. 14 Series II HN03, 30 g./l.;HC1, 5 g.[l 104 5 12 0.07 Series III. HNO3, 47. 2 g./1.; H01, 2. 3 g./1 104 5 13 O. 046

Normally, Grade K tinplate is required only on those tinplate products having a tin coating weight of 0.5 pound or more per base box (217.78 sq. ft. of sheet). By use of the invention, Grade K tinplate can be produced for acid tinplate of these coating weights, as well as for tinplate somewhat below 0.5 pound per base box.

The nitric acid concentration of the pickling bath may range from 2 to 5.5 weight percent (calculated as 100% HNO while the hydrochloric acid concentration should be Within the limits of from 0.2 to 1.7 weight percent (calculated as 100% HCl). Preferably, nitric acid is maintained at from 3 to 5 weight percent, and hydrochloric acid is held within the range of from 0.4 to 0.8 weight percent.

For best performance, temperature of the bath should be kept above about 70 F. While temperatures considerably above 100 F. may be used, the temperature should be kept below that at which visible ethcing of the strip occurs, and below that at which acid would be lost by excessive volatilization.

When less than 0.2% hydrochloric acid is used there is a danger of overpickling due to insuflicent inhibiting action by the hydrochloric acid. Overpickling produces a roughened surface on the steel strip, which in turn is displayed on the tin-coated stock, even after fusing the tin coating. On the other hand, use of excessive amounts of References Cited UNITED STATES PATENTS 1,859,736 5/1932 George. 2,266,330 12/ 1941 Nachtman 204-28 2,940,837 6/1960 Acker et al 156-18 XR FOREIGN PATENTS 5,693 1903 Great Britain.

JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner US. Cl. X.R. 204-32, 34

UNITED STATES PATENT OFFICE CERTIFICATE OF CORREGTION Patent No. 3,468,768 September 23, 1969 Stewart E. Rauch, Jr.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Columns 3 and 4, in the table, second column, line 1 thereof, "H 80 5 g./l." should read H 80 5O g./1.

Signed and sealed this 20th day of October 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER,

Edward M. Fletcher, J r.

Commissioner of Patcnjs Attesting Officer 

